A typical application for a chain tensioner is the maintenance of tension in an automobile engine timing chain. In an engine timing drive, an endless transmission chain, for example, a roller chain, is driven by a crankshaft sprocket and in driving relationship with one or more camshaft sprockets. The chain travels in sliding relationship over a movable chain guide which is urged against the chain by the plunger of the tensioner in order to maintain tension in the span of the chain that travels from the crankshaft sprocket toward a camshaft sprocket.
The plunger is biased in the protruding direction at least in part by oil supplied under pressure to a high pressure oil chamber formed by the plunger and the tensioner housing. Movement of the plunger in the retracting direction is controlled by hydraulic damping effected by leakage of oil through a restricted space between the outer peripheral surface of the plunger and the cylindrical inner wall of the plunger-accommodating hole. On engine start-up, oil pressure is not yet built up in the oil chamber of the tensioner, and, if the engine has not been operated for a long time, oil in the oil chamber may be depleted. Consequently, the damping function will not take place until oil pressure has been reestablished. Until oil pressure is reestablished, the plunger can retract excessively, causing the timing drive to generate a rattling noise.
To address the aforementioned problem, it is known to provide a chain tensioner with a ratchet mechanism composed of a plurality of concave grooves around the plunger and a groove-engaging member provided in the tensioner housing. The groove-engaging member restricts movement of the plunger in the retracting direction. Such a tensioner is disclosed in laid-open Japanese Patent Application No. 3929680, published on Jan. 9, 2002.
The prior art ratchet mechanism not only restricts retracting movement of the plunger on engine start-up, but also restricts retraction of the plunger when excessive chain tension occurs during normal engine operation, due to temperature changes and other causes. In this case, seizing of the plunger can occur, resulting in an increased load on the chain and the generation of excessive noise.
The problems arising from excessive chain tension during normal engine operation can be alleviated to some extent by providing for a predetermined amount of backlash in the ratchet mechanism of the tensioner. The predetermined amount of backlash is set to correspond to a presumed maximum movement of the plunger in the retracting direction caused by the excessive chain tension. However, as the amount of allowable backlash increases, the amount of rattling noise generated on engine start-up also increases.
In the prior art chain tensioner the grooves on the outer peripheral surface of the plunger have forward surfaces, i.e., surfaces facing in the direction in which the plunger protrudes, that have a gradual slope with a constant angle of inclination. These surfaces are engaged by a resilient ring, whose diameter becomes larger as a result of the force exerted by a forward groove surface. The spring modulus of the ring, however is such that the force exerted axially on the plunger to expand the resilient ring increases as the ring expands. Consequently the advancing movement of the plunger depends on the plunger position relative to the ring, the damping response of the tensioner becomes uneven, the plunger does not advance smoothly, and “flip-flop” noises are generated.
In another kind of ratchet tensioner, the member that engages ratchet teeth on the plunger is a pawl that is pivotally mounted on the tensioner housing. Because a part of the pawl extends outside the tensioner housing, the overall size of the tensioner is increased. The pawl mechanism also increases the complexity of the tension, and makes installation and removal of the tensioner more difficult. Furthermore, designing the ratchet mechanisms of these prior art chain tensioners so that their characteristics meet operating conditions requires modification of the entire tensioner, thereby increasing manufacturing costs.